Cryogenics38 (1998) 75–78 1998 Elsevier Science Ltd. All rights reserved

Printed in Great BritainPII: S0011-2275(97)00113-6 0011-2275/98/$19.00

Fatigue behaviour of compositesG. Hartwig*, R. Hu

¨bner*, S. Knaak* and C. Pannkoke†

*Forschungszentrum Karlsruhe, Institut fur Materialforschung II, Postfach 3640, D-76021 Karlsruhe, Germany†Fraunhofer Institut fur Angewandte Materialforschung, Lesumer Heerstr. 36, D-28717 Bremen, Germany

Received 12 September 1996; revised 7 July 1997

An important design parameter for cyclically loaded structures (e.g. transport vessels)is the fatigue endurance limit. The cryogenic fatigue behaviour with different types offibres and matrices has been investigated. The main emphasis it put on the behaviourof fibre dominated properties. It is surprising that the fatigue strength even of unidirec-tional fibre composites is strongly influenced by the matrix type. This will be discussedfor carbon fibre composites with thermoplastic and duroplastic matrices under tensileand shear loading.

For crossplies (with non-woven fabrics) the interaction between laminates controlsthe fatigue behaviour. The interaction depends on the matrix type and is different fortensile and shear loading. 1998 Elsevier Science Ltd. All rights reserved

Keywords: fatigue; low temperatures; fibre composites; tensile strength; shearstrength

At many cryogenic applications fibre composites areexposed to cyclic loading. Their fatigue endurance limit istherefore an important design parameter. Transport vessels,structural elements of pulsed SC-magnets, and wind tunnelsare some examples.

The degradation by fatigue cycling depends on the fol-lowing parameters:

• fibre type and arrangement;• matrix type;• interfacial bond;• mode of loading

– threshold tension or threshold compression,– tension–compression,– shear or torsion,– bending.

There are several general results, characteristic of thefatigue behaviour of fibre composites:

• the frequency of fatigue cycling at cryogenic tempera-tures can be chosen much higher than at RT withoutremarkable heating of specimens (the mechanical lossfactor tand is lower by a factor of< 10 at 4 K, andabout 5 at 77 K compared to RT; in addition the heatdiffusion is higher at low temperatures);

• fatigue strength even of fibre dominated properties isstrongly influenced by the matrix type;

• alternating loading leads to a shorter fatigue life thanfor threshold cycling;

• Young’s modulus of UD composites is nearly

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unchanged by fatigue cycling – this is not always truefor crossplies;

• multiaxial loading (e.g. tension–torsion) reduces mod-uli and fatigue life drastically;

• the fatigue behaviour of the pure matrix is usually notreflected in the fatigue behaviour of a composite;

• under similar conditions, tensile fatigue strength of car-bon is highest, followed by Kevlar and glass; ceramiccomposites exhibit the lowest fatigue strength.

The influence of the fibre type will be discussed by theexample of unidirectional and crossplied composites. Thefatigue behaviour is studied on composites with the samematrix but different fibres.

Usually, the influence of the weak matrix on fibre domi-nated properties is thought to be of less importance. How-ever, there are composites which are rather sensitive to thematrix. The influence of the matrix even on fibre dominatedproperties is largest when fibres of low stiffness (glass) orof low transverse strength (carbon or Kevlar) are applied.It is astonishing that the fatigue endurance limit of unidirec-tional carbon fibre composites loaded in fibre direction isgreatly influenced by the weak matrix. It is further astonish-ing that brittle matrices (highly crosslinked epoxies) yielda higher fatigue limit than a ductile thermoplastic matrix(e.g. PEEK, PC). This different behaviour arises from dif-ferent fracture mechanisms in the matrices.

This behaviour is also reflected in crossplies (lay-upplies). But there it might be overlapped by a destructiveinteraction between 0° and 90° layers. Cracks induced inthe 90° layer might destroy the load-bearing 0° layers of

Fatigue behaviour of composites: G. Hartwig et al.

Figure 1 S–N curves of unidirectional fibre composites with similar epoxy matrices at 77 K: (a) absolute values; (b) normalizedvalues s/sUT

carbon fibre composites. The influence of the matrix onthe static and fatigue strength is studied on carbon fibrecomposites under tensile and shear (torsion) loading. Ten-sile threshold and shear threshold loading at a frequency of30 to 80 Hz has been applied. The fatigue endurance limitsD is defined as the upper load amplitude withstanding 107

load cycles.

Dependence on fibre type

Unidirectional (UD) composites; tensile loading

Composites with the following fibre types have been inves-tigated:

• carbon – T300; HTA; AS4;• glass – E-glass;• ceramic – Al2O3 (Sumitomo Chemical Co.);• Kevlar 49.

The fatigue behaviour of composites with different fibretypes (carbon AS4; HTA; E-glass, ceramic Al2O3 and Kev-lar 49) and similar epoxy matrices are plotted inFigure 1ain so-calledS–N curves (stress–load cycles diagram). Thehighest strength is achieved with carbon fibre composites.In Figure 1b the relative values are plotted, normalized tothe static strengthsUT. It can be seen that the lowest degra-dation occurs for Kevlar fibre composites. Some data aregiven in Table 1.

Crossplies (0°, 90°); tensile loading

SomeS–N curves of crossplies with different fibre typesbut the same matrix are plotted inFigure 2a and b. Thecrossplies consist of sandwiched layers (no wovenlaminates). The Kevlar composites are quasi-isotropic withlayers every 45°. The trends are similar to those of UDcomposites. Some data are given in Table 2.

Table 1 Static tensile properties and fatigue endurance limit sD of unidirectional composites with similar epoxy matrices at 77 K

Fibre sUT (MPa) eUT (%) E (GPa) sD (MPa) sD/sUT (%)

Carbon (AS4) 2440 ± 90 1.6 ± 0.1 151 ± 6 1250 51Glass 1550 ± 90 5 ± 0.2 46 ± 2Ceramic (Al2O3) 2000 ± 75 1.7 ± 0.1 122 ± 4 640 32Kevlar 49 1255 ± 80 1.2 ± 0.1 103 ± 4 816 65

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Influence of the matrix

Composites having a poor transverse strength are sensitiveto microcracks induced in the matrix. The formation ofmicrocracks at fatigue loading is different for differentmatrix types, and so is the fatigue endurance limit. Thefollowing matrices are applied:

• brittle epoxy (EPb) from Ciba, LY 556/HY 917;• modified epoxy (EP mod) from Ciba, V 913;• standard epoxy (EP) from Shell, E162/E113;• semi-flexible epoxy (EPf) from ICI, 977-2;• thermoplastic PEEK (polyetheretherketone) or PC

(polycarbonate).

The most contrasting matrices are EPb and the thermo-plastic PEEK (or PC), the latter being rather ductile evenat cryogenic temperatures. Their influence is shown inFig-ure 3a for unidirectional carbon fibre composites by strainlife diagrams. Since matrix and fibres are loaded in parallelfor UD composites, the strain is the appropriate parameterinstead of stress. This is true if no modulus degradationoccurs during fatigue cycling. Separate investigationsrevealed that the modulus is nearly unchanged till fractureoccurs. The highest strain fatigue endurance limit is achi-eved with the brittle matrix EPb, and the lowest with PEEK(or PC)1. During loading many transverse microcracks areinduced in the EPb matrix. The PEEK matrix tends to aformation of longitudinal cracks which reduces the sheartransfer between neighbouring fibres more than the trans-verse cracks in the EPb matrix2. Shear transfer is importantespecially when fibre breakage occurs.

The degradation of the shear transfer strength can bestudied more directly on shear loading of 90° UD com-posites. As is obvious fromFigure 3b, the composites witha PEEK matrix again degrade more strongly. The staticshear strength depends mainly on the matrix.

Fatigue behaviour of composites: G. Hartwig et al.

Figure 2 S–N curves of crossplied composites with different fibre types and an EP matrix at 77 K: (a) absolute values; (b) relativevalues s/sUT

Table 2 Static tensile properties and the fatigue endurance limit sD of crossply composites with similar epoxy matrices at 77 K

Fibre sUT (MPa) eUT (%) E (GPa) sD (MPa) sD/sUT (%)

Carbon (HTA) 980 ± 150 1.2 ± 0.1 83 ± 9 665 68Glass 910 ± 40 4.7 ± 0.3 24 ± 2Ceramic (Al2O3) 613 ± 24 1.1 ± 0.1 54 ± 4 250 41(Kevlar 49a) (345 ± 14) (1.2 ± 0.1) (38 ± 1) (173) (50)

a Quasi-isotropic lay-up

Figure 3 (a) Strain life curves of UD carbon fibre composites under tensile loading. (b) S–N curves of 90° UD carbon fiber compositesunder shear loading (torsion of tubes)

Figure 4 S–N curves of carbon fibre crossplies with different matrices and illustrations of interactions

Cryogenics 1998 Volume 38, Number 1 77

Fatigue behaviour of composites: G. Hartwig et al.

Figure 5 Relative shear modulus versus load cycles N for 0°/90° composites with a rigid epoxy matrix and a toughened epoxy matrix

Crossplies (0°, 90°); tensile loading

The results observed on UD composites can be transposedto crossplies, if no interaction between 90° and load bearing0° layers occurs. Again the matrix plays a dominant role;but in a different manner as discussed above. Dependingon the matrix type, three types of interactions have beenobserved for carbon fibre crossplies:

• destructive interaction (with EPf matrix);• neutral interaction, delamination (with EPb matrix);• constructive interaction (with PEEK matrix).

The destructive interaction occurs because of the weaktransverse strength of the anisotropic carbon fibres. Trans-verse cracks, induced in the 90° layers, propagate anddestroy load bearing 0° fibres.

Delamination of 0° and 90° layers avoids interaction, butthe composite is damaged. With PEEK the microcracks, inthe 90° layers, are deflected before reaching the 0° layers.

Results and a sketch of interaction processes are shownin Figure 4. Despite different static strengths the fatiguelimit is similar.

It is worth mentioning that a thin fibre glass laminatebetween the 0° and 90° carbon fibre layers acts as a crackstop and avoids reduction of strength3.

Crossplies ( ± 45°); shear loading

The shear strength of± 45° crossplies is fibre dominated,but its value is much lower than the tensile strength of UDcomposites4. The shear stress can be divided into tensileand compressive stress components along the+45° and−45° layers, respectively. Only half of the layers is loadedin tension and the compressive strength is nearly half thetensile strength. But there is, however, a comfort. Nearlyno degradation occurs by fatigue cycling since only lowload amplitudes are applicable.

Modulus degradation

Further investigations considered the degradation of moduliunder fatigue cycling. For fibre dominated moduli (e.g.EII

of UD composites) nearly no degradation (about 5%) hasbeen observed. This is also true for the shear modulusGof ± 45° crossplies (fibre dominated). For (0°, 90°) crosspl-ies with fibres of low stiffness (e.g. fibre glass) a degra-

78 Cryogenics 1998 Volume 38, Number 1

dation ofE has been observed. The matrix dominated 90°layers, which in this case contribute remarkably to theE-modulus, degrade strongly, and thus the total stiffness. Thisis not true for (0°, 90°) carbon or Kevlar fibre crosspliesbecause of the great longitudinal stiffness of these fibres.Their transverse stiffness, however, is low, and thus the 90°layers are of much lower influence than the load-bearing,non-degrading 0° layers. No remarkable modulus degra-dation has been observed on (0°, 90°) carbon or Kevlarfibre crossplies.

An example is given for the shear modulus of carboncrossplies inFigure 5. GN is the modulus atN load cyclesand G0 is the initial modulus. The crosses mark thefatigue failure.

Conclusions

• The fatigue behaviour of fibre dominated propertiesdepends strongly on the fibre type.

• Even fibre dominated properties of carbon fibre com-posites are strongly influenced by the matrix type.

• Carbon fibre crossplies may fail because of destructiveinteractions of the 90° and the load bearing 0° layers.The reason is the low transverse strength of the aniso-tropic carbon fibres and the sensitivity to transversematrix cracks.

• Modulus degradation by fatigue cycling is negligible forUD composites.

• Modulus degradation of crossplies (0°, 90°) is negli-gible if the load bearing 0° layers have a much higherstiffness than the 90° layers (e.g. anisotropic fibres: car-bon or Kevlar fibres). Fibre glass has a low stiffness,and moduli of crossplies degrade.

References

1. Pannkoke, K., Static and fatigue properties of UD carbon fiber com-posites at 77 K.Adv. Cryo. Eng. Mater., 1994,40, 1025–1034.

2. Hartwig, G., Hu¨bner, R. and Knaak, S., Fatigue behavior of polymersand composites at cryogenic temperatures.Adv. Cryo. Eng. Mater.,in press.

3. Hartwig, G., Reinforced polymers at low temperatures.Adv. Cryo.Eng. Mater., 1982,28, 179–189.

4. Hartwig, G. and Hu¨bner, R., Fatigue behaviour of crossply carbonfiber composites at cryogenic temperatures.Adv. Cryo. Eng. Mater.,in press.